I finally got this rune, and tested it out.
I can confirm that effect is triggered, and that it will trigger off of large groups of normal monsters as well as bosses and elites, although it seemed to take more than 5 monsters before the effect was triggered. Exactly how many more I could not be certain of, because the monsters were already dying by the time I saw it trigger.
It seems to attack quite a bit faster than normal, so I'd say the 35% attack speed is a 35% bonus, rather than 35% of normal attack rate.
His damage appeared to be roughly 3x normal (I was seeing hits with the buff around 300, with crits around 450, and hits without the buff were in the 75-125 range), so again, the "200% Physical damage" appears to be +200% additional damage.
The buff lasts about 15 seconds.
All in all, this is a pretty nice buff.
It does not seem to matter how fast you beat on them to determine the quality of the overall drops. The most important thing is to actually kill them before they open a portal and escape. The last drops upon death (and possibly right before death) are magic items, as opposed to gold, so it is well worth making certain you kill them.
There are different types of treasure goblins, and they seem to have different qualities of loot.
I encountered a Treasure Pygmy during Act 3 of normal, and, despite not killing him particularly quickly (certainly not as fast as I had killed some early treasure goblins), I had the extreme fortune of having 3 rare items drop from him. I'm not certain if this is simply an extremely lucky coincidence, or if the quality of the drops was influenced by the particular monster title.
Edit: I just encountered a Treasure Goblin much later in Act 3, and he dropped 4 regular magic items (no rares), even though I beat him down much faster than the Treasure Pygmy. This seems to corroborate the idea that the Treasure Pygmy may simply be a rarer, better version of the Treasure Goblin.
Edit: After quite a few more tests, ranging from chasing the Treasure Goblin (of a number of varieties) around, to beating them down before they could move more than a few yards, to killing them moments before they step through a portal, there is absolutely no discernible difference in the quantity or quality of items they drop. They always drop 1-4 magic items, plus gold, gems, and crafting pages/tomes (if you are high enough level for gems and/or crafting pages/tomes to drop).
Best Answer
With the introduction of the bounty system came a way to be aware of most of the rare spawns in a game. For example, this also includes places like the Caverns of Frost in Act III where people farm for one of the components of the Staff Of Herding.
Prior to the expansion, these rare spawns would have to manually be sought out. Snitchler is one of these such rare spawns. However, Snitchler is not that rare because there are no associated achievements to Snitchler. I killed him last night, it took me 23 new games to get the bounty, and I got no achievement. When examining the list of rare spawn enemies to kill to complete Snitchler was not in the list.